Stripping is a process used to remove volatile components from water. The basic concept is to bring the contaminated water into intimate contact with a stripping gas, frequently air, so that the volatile compounds undergo a phase change from liquid to vapor and are carried away by the stripping gas. A number of interrelated design factors affect the stripping efficiency: the Henry's law coefficient, the stripping gas:water volume ratio, the contact time and mass transfer rate. The gas:water volume ratio used depends on the volatility of the component to be removed, its concentration in the feed water and the physical attributes under which the contact is carded out. It is typically in the range 50:1-500:1 or more. The component being removed is, therefore, diluted by this amount when it is transferred from water to the gas. When other factors are constant, a high gas:water volume ratio provides a high percentage of component removal from the water, but creates large volumes of gas contaminated with dilute concentrations of component. A low gas:water volume ratio may provide insufficient dilution of the component in the gas to maintain a good driving force for mass transfer. Under optimum conditions, transfer of the organic compound from the water to the gas can be very efficient and removal rates up to 99.99% can be achieved.
The design of a stripping system depends on the nature of the feed stream to be treated and the desired composition of the water stream exiting the stripper. In comparing different processes, consideration should always be given to the advantages of batchwise and continuous operation. Industrial operations that generate large streams (more than 10,000 gpd) of relatively uniform concentration justify employing a continuous stripping operation because the high cost of continuous equipment and instrumentation (process control devices) is outweighed by the advantages of a lower unit investment, operating cost, and uniform quality. Industrial operations that generate small (100-10,000 gpd), intermittent streams or streams with varying composition or components do not employ continuous stripping because either the high cost of continuous equipment and instrumentation is not justified or the streams themselves are not capable of treatment in continuous mode. In such cases, treatment of the stream by batchwise stripping is preferred and far more efficient.
To date, however, the benefit of the combined separation by a stripper-membrane unit has not been available to industrial operations that generate small, intermittent streams, or streams with varying compositions or components. Such potential users may not have the resources of large plants to pool, adjust or pretreat feed streams to bring them within tight starting parameters for treatment in continuous operations. There exists, therefore, a need for separation equipment that offers flexibility from a standard design and that is within the economic and technical resources of relatively small companies.